Similar to our previous TF-QKD experiment in the lab [1]}, [2]}, Alice and Bob respectively adopt a ultra stable continuous wave (CW) laser with a linewidth less than 1 Hz as the light source. First, they use a commercial continuous wave laser (NKT E15) with a linewidth of about 1 kHz at the center wavelength of 1550.12 nm as the seed laser. Then Alice locks her seed laser to a 5-cm-long ultra-stable cavity with a finesse around 400000, and Bob locks his seed laser to a 10-cm-long ultra-stable cavity with a finesse around 400000, using the Pound-DreverHall (PDH) technique [3]}, [4]}, to suppress their line-width from about 1 kHz to less than 1 Hz. Since the ultra-stable cavity is machined by ultra-low-expansion (ULE) glass, the resonant frequency of each cavity is different, after the seed laser is respectively locked to their cavity, finally, they additionally need to calibrate the frequency/wavelength difference of the two stable laser sources. Compared to the experiments in the laboratory [1]}, [2]}, we change the way the twin-field light sources works to decrease the crosstalk noise. We calibrate the frequency difference of the two independent light sources intermittently instead of continuous frequency locking. We first calibrate the wavelength difference of the two independent light sources before the experiment, with strong CW reference light in the fiber; Then when the experiment is running, the strong reference is blocked, and the wavelength between the two light sources are left to free drifting. With stable light sources, the wavelength difference will be small for the experiment between the wavelength calibration cycles.
